Lightweight construction element and method for producing the same

ABSTRACT

The invention relates to a lightweight construction element having an inner framework structure of light metal composed of a plurality of extruded hollow sections ( 1, 2, 3 ) joined to one another in a flat configuration, the lightweight construction element having a circumscribed circle with a diameter of at least 300 mm and a wall thickness of at most 0.5% of this value.

The present invention relates to a lightweight construction elementhaving an inner framework structure of light metal. The invention alsorelates to a method for producing the lightweight construction element.

The use of light metals is one of the greatest challenges in theconstruction of locomotion means, especially automobiles, sinceminimization of weight is one of the most effective methods of reducingfuel consumption.

Against the background of a cost-to-benefit comparison of differentlight metals, it is clear that the manufacturing costs increasedrastically with increasing weight savings by the use of such materials.Thus lightweight construction can only be achieved economically if itbecomes possible to compensate for the associated higher material costsby more favorable production processes and in particular by more sparinguse of materials.

Lightweight construction elements having an inner framework structure oflight metal material have proved advantageous for the purpose of thebest possible ratio between weight and load-bearing ability or strength.Such lightweight construction elements can be produced economically byextrusion.

As regards the extrusion process, however, the relative ratio betweenthe size of the section to be pressed and the size of the extrusionpress and especially of the chamber diameter is critical for thematerial flow of a given material. For example, from the AluminumTextbook published by Aluminium-Verlag of Düsseldorf, 15^(th) Edition1996, Vol. 2, p. 103, it is known that, in the extrusion of hollowsections made of pure aluminum or of AlMgSi alloys and having uniformwall thickness, a section having a section circle diameter of 450 mm andobtained by extrusion in an 80-MN extrusion press can have a minimumwall thickness of 5 mm, whereas a section having a section circlediameter of 50 mm and obtained by extrusion in a 10-MN extrusion presscan have a minimum wall thickness of 1 mm. This shows that largelightweight construction elements can be produced only with relativelythick wall thicknesses by extrusion, meaning higher production costsand, because of the greater component weight, also a negative influenceon the fuel consumption of a vehicle containing this component.

Against this background, the object of the present invention is toprovide a lightweight construction element having an inner frameworkstructure of light metal, wherein the wall thickness is smaller than inconventional lightweight construction elements produced by extrusion.

This object is achieved according to the invention by the fact thatthere is taught a lightweight construction element having an innerframework structure of light metal, comprising a plurality of extrudedhollow sections joined to one another, the lightweight constructionelement having a circumscribed circle with a diameter of at least 300 mmand a wall thickness of at most 0.5% of this value. In a particularlypreferred embodiment of the invention, the wall thickness is at most0.35% of the diameter of the circumscribed circle of the lightweightconstruction element.

According to the invention, therefore, by extruding individual hollowsections and joining the hollow sections in a planar configuration,there can be obtained a lightweight construction element of practicallyany desired size with a wall thickness, which construction element,because of the technical limitations of the extrusion process, cannot bemanufactured in monolithic form or can only be manufactured with muchgreater linear density or greater wall thickness once its size exceeds acertain value (generally a circumscribed circle with a diameter oflarger than 300 mm).

As the Applicant has surprisingly found, the extruded hollow sections ofthe inventive lightweight construction element can be joined by frictionstir welding. Heretofore those skilled in the art have assumed that thefriction stir welding technique requires that the workpieces to bewelded each have a wall thickness of at least 1.6 mm (most recentlystated in a contribution by the Alusuisse Co. at the 2^(nd) TechnicalConference on “Advances in Lightweight Automotive Engineering”,Stuttgart, 6 to 7 Nov. 2001). Friction stir welding (FSW) had alreadybeen developed almost ten years ago (see European Patent B 0615480).Nevertheless, it is not yet one of the standard joining techniques inthe automobile industry, where only resistance welding, inert-gaswelding and laser (hybrid) welding have been used heretofore as thermaljoining techniques.

It is particularly advantageous in friction stir welding—in contrast toconventional welding techniques—that welding of the two workpieces takesplace below the liquidus temperature of the materials to be welded, andso no appreciable risk of development of pores and hot cracks exists.Moreover, even alloys that are difficult or impossible to melt as wellas aluminum/magnesium composite elements can be welded with frictionstir welding, an accomplishment that is difficult or even impossiblewith the conventional welding techniques. Thus entirely newpossibilities for the production of composite components are created bythe friction stir welding technique.

As an alternative to friction stir welding, the individual hollowsections can be joined by adhesive bonding, which has the advantage inparticular that the hollow sections to be joined are subjected to onlyslight thermal stress, whereby development of pores and hot cracks isavoided.

To ensure that the hollow sections composing the lightweightconstruction element can be joined, they can be provided withappropriate elements in the form of ridges, hooks or grooves, so thatthe elements in the form of ridges, hooks or grooves of adjacent hollowsections have corresponding shape and can overlap in a planarconfiguration of the hollow sections, in order to be able, together withthe adjacent zones of the sections, to withstand the forces occurringduring friction stir welding.

As an alternative to this, it may also be possible to avoid the use ofhollow-section elements in the form of ridges, hooks or grooves, inwhich case the hollow sections are joined only along an abutting edge.To ensure that no deformations of the hollow sections are caused duringfriction stir welding, the forces occurring at this time must beabsorbed by an appropriate fixture, such as an inner mandrel. Avoidingthe use of elements in the form of ridges, hooks or grooves can beregarded as advantageous, since this contributes to economy of materialsand thus to reduction of costs and weights.

The individual hollow sections may be made of aluminum, magnesium,titanium or alloys thereof. By joining hollow sections of dissimilarmaterials, it is advantageously possible to produce composite members.

In a particularly advantageous embodiment of the invention, alightweight construction element comprises a plurality of mutuallysymmetric, individual hollow sections. Hereby the costs of producing alightweight construction element can be greatly reduced by a smallernumber of tools and simplified logistics.

For production of the inventive lightweight construction element, hollowsections with a wall thickness of at most 0.5% of the diameter of thecircumscribed circle of the lightweight construction elementmanufactured therefrom are produced by extrusion. The extruded hollowsections are then joined in a planar configuration to form a lightweightconstruction element, in such a way that the lightweight constructionelement has a circumscribed circle having a diameter of at least 300 mm.Friction stir welding and adhesive bonding are preferably used forjoining the hollow sections.

The inventive lightweight construction element produced in this way ispreferably used as part of a load-bearing structure, for example in amotor vehicle.

The invention will now be explained in more detail on the basis ofpractical examples with reference to the attached drawings, wherein

FIG. 1 shows a sectional view of an inventive lightweight constructionelement having an inner framework structure comprising three joinedindividual hollow sections;

FIG. 2 shows, in the form of a Wöhler diagram, the behavior of thestress amplitude A [MPa] as a function of the number N of load cycles ofa lightweight construction element manufactured by friction stir welding(curve a) and by laser welding (curve b);

FIG. 3 shows examples of the joining points of adjacent hollow sections.

Referring first to FIG. 1, wherein there is illustrated a sectional viewof an inventive lightweight construction element having an innerframework structure, the lightweight construction element is composed ofthree hollow sections 1, 2, 3 in a planar configuration. The two outerhollow sections 1, 3 have mutually symmetric shape, in that one of thetwo hollow sections has merely been rotated by 180° around itslongitudinal axis relative to the other hollow section. The two outerhollow sections 1, 3 are provided with ridge-shaped connecting elements4, 5, while middle hollow section 2 is provided with ridge-shapedconnecting elements 6, 7 of shape complementary thereto. During joiningof the hollow sections, the ridge-shaped connecting elements of adjacenthollow sections are brought into mutual contact and are joined bytechniques such as friction stir welding. The forces developed duringfriction stir welding are absorbed by the ridge-shaped connectingelements and the zones 8, 9 of the hollow sections adjacent to them,whereby undesired deformations of the hollow sections can be avoided.The enlarged detail shows how ridge-shaped connecting elements 6, 7 ofthe two hollow sections 2, 3 are brought into contact via theircomplementary shapes.

The lightweight construction element illustrated as an example in FIG. 1is made of aluminum hollow sections and, for a wall thickness of about 1mm, has a circumscribed circle with a diameter of about 500 mm. Thejoined individual hollow sections have a circumscribed circle with adiameter of about 170 mm.

By comparison with the manufacture of a corresponding lightweightconstruction element from two equally large individual hollow sections(circumscribed circle with a diameter of about 250 mm), in which casethe wall thickness achievable by the extrusion technique was 2 mm andthe individual hollow sections were welded together by laser welding,the weight savings achieved in the inventive lightweight constructionelement was about 15%.

In a particularly advantageous manner, the endurance limit of thelightweight construction element produced can be greatly increased inthe case of hollow sections joined by friction stir welding. Forcomparison of the endurance limit of lightweight construction elementsproduced by laser welding and by friction stir welding, appropriatelymanufactured lightweight construction elements were subjected to asinusoidally increasing and decreasing tensile stress at various loadlevels. The result is illustrated in FIG. 2, which shows, in the form ofa Wohler diagram, the behavior of the stress amplitude A [MPa] versusthe number N of load cycles of similar lightweight construction elementsmanufactured by friction stir welding (curve a) and by laser welding(curve b).

As is evident from FIG. 2, a laser-welded lightweight constructionelement subjected to a high load level of 75 MPa can be expected to failalready at about 33,000 load cycles, whereas such failure is notexpected until about 240,000 load cycles in the case of afriction-stir-welded lightweight construction element. For high load,therefore, this means that the stress and strain endurance of thefriction-stir-welded lightweight construction element is about 7.4 timesgreater than that of the laser-welded lightweight construction element.For the case of a low load level of 47 MPa, failure of the laser-weldedlightweight construction element takes place at about 2 million loadcycles, whereas it does not occur until about 5 million load cycles inthe friction-stir-welded lightweight construction element. For low load,therefore, this means that the stress and strain endurance of thefriction-stir-welded lightweight construction element is about 2.5 timesgreater than that of the laser-welded lightweight construction element.

It is also evident that a fracture of the laser-welded lightweightconstruction element is generally located in the weld, starting from theupper side of the weld and from hydrogen pores, whereas fractures of thefriction-stir-welded lightweight construction element are located in thebase metal and start from notches in the section, or in other wordsextrusion marks or surface irregularities.

FIG. 3 shows, in sectional view, various shapes of joining points of thehollow sections. The hollow sections shown in Case I are each providedwith hook-shaped connecting elements 10, 11 of corresponding shape. Tojoin the hollow sections, the hook-shaped connecting elements areengaged in one another (right diagram) and then are joined at thecontact faces, for example by friction stir welding. The mechanicalpressure forces exerted by the welding mandrel on the hollow sectionsare absorbed by hook-shaped connecting elements 10, 11 and the adjacentzones 18, 19 of the sections, thus counteracting deformation of thehollow sections.

In Case II of FIG. 3, one hollow section is provided with theridge-like, terrace-shaped connecting elements 12, 13, while the hollowsection to be joined thereto is provided with ridge-like connectingelements 14, 15 having a terrace shape corresponding thereto. To jointhe hollow sections, the ridge-like connecting elements are brought intocontact with one another and then are joined at the contact faces, forexample by friction stir welding. The pressure forces exerted on thehollow sections during friction stir welding are absorbed by theterrace-shaped connecting elements and the adjacent zones 20, 21 of thesections.

In Case III of FIG. 3, the hollow sections are provided with flatabutting faces 16, 17 of corresponding shape. For joining, the abuttingfaces 16, 17 are brought up against one another and joined at thecontact faces. The pressure forces acting on the sections duringfriction stir welding must be absorbed by an appropriate fixture, suchas an inner mandrel, in order to prevent deformations of the hollowsections.

1. A lightweight construction element having an inner frameworkstructure of light metal, comprising a plurality of extruded hollowsections joined to one another in a planar configuration, thelightweight construction element having a circumscribed circle with adiameter of at least 300 mm and a wall thickness of at most 0.5% of thisvalue.
 2. A lightweight construction element according to claim 1,characterized in that the wall thickness is at most 0.34% of thediameter of the circumscribed circle of the lightweight constructionelement.
 3. A lightweight construction element according to claim 1,characterized in that the wall thickness of the lightweight constructionelement is 1.5 mm.
 4. A lightweight construction element according toclaim 1, characterized in that the wall thickness of the lightweightconstruction element is 1 mm.
 5. A lightweight construction elementaccording to claim 1, in which the hollow sections are joined byfriction stir welding.
 6. A lightweight construction element accordingto claim 1, in which the individual hollow sections are joined byadhesive bonding.
 7. A lightweight construction element according toclaim 1, in which the hollow sections are provided with a connectingelement having the form of ridges, hooks or grooves suitable forabsorbing the forces occurring during joining.
 8. A lightweightconstruction element according to claim 1, in which the hollow sectionsare made of aluminum, magnesium, titanium or alloys thereof.
 9. Alightweight construction element according to claim 8, in which thehollow sections are made of dissimilar light metals or light-metalalloys.
 10. A lightweight construction element according to claim 1,composed of mutually symmetric individual hollow sections.
 11. A methodfor producing a lightweight construction element according to one of thepreceding claims and comprising the following steps: (a) extrusion ofhollow sections with a wall thickness of at most 0.5% of the diameter ofthe circumscribed circle of the lightweight construction elementmanufactured therefrom, (b) joining of a plurality of hollow sections ina planar configuration to form a lightweight construction element, whichhas a circumscribed circle with a diameter of at least 300 mm.
 12. Amethod according to claim 11, in which the hollow sections are joined byfriction stir welding.
 13. A method according to claim 11, in which thehollow sections are joined by adhesive bonding.